1. Field of the Invention
The present invention relates to a laser piercing method, a laser processing nozzle, and a laser cutting apparatus applied in the laser cutting of a cut work, such as a steel plate.
2. Description of the Related Art
In cutting, for example, a thick steel plate (a steal plate with a thickness of 6 mm or greater) by laser, usually the intended cutting operation commences after executing a piercing operation.
Generally, a piercing operation, as shown in FIG. 13, consists in applying a laser beam 2 from a cutting nozzle 1 to the cut work 3, such as a steel plate, heating the cut work 3, and at the same time, by supplying an assist gas 4 which is coaxial to the laser beam 2, forming the piercing hole 5 by removing the molten metal 6 form the heated part of the cut work 3 by the kinetic energy of the assist gas 4. When implementing the piercing operation, part of the molten metal 6 accumulates around the piercing hole 5, and one part is scattered to places separated from the piercing hole 5.
As an assist gas 4, usually oxygen is used. By using oxygen gas when the piercing operation is executed, there is the advantage that the piercing operation can be executed with high efficiency because a high energy is obtained by the oxidation of the molten material due to the oxygen gas. In addition, in the application condition of the laser beam 2 during piercing, the pulsed oscillation is generally equal to or less than 100 Hz, but by increasing the output of the laser beam 2 in a state of continuous oscillation, it is possible to form the intended piercing hole 5 in a short time, and the piercing operation time can be shortened.
However, when increasing the output of the laser beam 2, the problems occur that:
a. the piercing hole 5 diameter increases PA1 b. the blowing away the molten metal greatly increases, PA1 c. due to b, the cutting nozzle 1 and the focusing lens are sometimes damaged, PA1 d. the sputter adhering to the cut work 3 increases, and PA1 e. due to d, incomplete cutting is produced when the intended cutting commences.
In addition, in consideration of these problems, in recent years attempts have been made to execute the piercing operation at high speed by raising the peak output of the pulse of the laser beam. However, because the problem of the adhesion of sputter to the lens and nozzle, no basic solution has been found for this problem.
Furthermore, attempts have been made to prevent the blowing away of molten metal and adhering of sputter by controlling the laser beam output during the piercing operation, but the piercing speed follows the control speed, and there are limits to increasing the speed.
Because of the above described situation, it is unavoidable that the piercing operation take a long time for a thick plate in particular. Furthermore, it is difficult to obtain the intended shape of the piercing hole. In order to ensure the passage of the assist gas and the removal of molten metal when switching from piercing to cutting, as well as stabilize the cut at the beginning of the cutting, it is desirable that the cross section and the inner surface of the piercing hole be a perfect circle having a diameter close to the external shape of the laser beam and be smoothly formed. However, in the piercing operation for a thick plate, because the removal of molten metal from the piercing hole during formation is even more difficult, there are many cases in which the diameter of the piercing hole may become extremely large and the shape of the cross section become distorted, and due to the adhering of dross, the inner surface becomes extremely irregular. Thus, when switching from piercing to cutting, the probability of producing an incomplete cut becomes higher. In addition, there is an increase in self burning (sudden local burning of the cut work 3, etc.) at the commencement of the cutting, and the cut becomes unstable. However, as the piercing time becomes long, the amount of heat input into the cut work becomes large during the piercing operation, the cut work becomes heated to a high temperature, and thereby when switching from piercing to cutting, there is excessive melting, and again an the probability of producing an incomplete cut becomes high.
In this manner, in the piercing operation for a thick plate, because it is difficult to obtain the intended shape of the piercing hole and because the cut work is heated to a high temperature due to the long piercing time thereby making excessive melting easily produced, the problems arise that incomplete cutting is easily produced, the cut at the commencement of cutting is unstable, and safely obtaining a high quality cut is not possible.
When executing the piercing operation with a fixed pulsed oscillation, as shown in FIG. 14, with the passage of the piercing time, that is, along with the progress of the piercing, it is clear that the amount of molten metal removed from the piercing hole per unit of time gradually decreases. The cause is that as the piercing hole gradually increases in depth as the piercing operation progresses, it is increasingly difficult for the molten metal inside the piercing hole being formed to be removed to the outside of the cut work. In fact, if the piercing is stopped midway, and the cross section of the piercing hole under formation is observed, it is possible to see that there is a rehardened layer, which was once melted but then hardened again, at the bottom of the piercing hole.
In this manner, if the removal of molten metal produced during the piercing operation is not smoothly removed, there may be an great increase in melting because the piercing operation time becomes lengthened, and in addition to this becoming a vicious cycle, influences the cutting operation which follows the piercing operation.